Distinct microbial communities across a climatically versatile summit in the Lesotho highlands

Abstract Most studies investigating the effects of climatological factors on microbial community composition and diversity focus on comparisons of geographically distinct environments (e.g., cold vs hot deserts) or across various temporal scales. Mountain regions provide unique environments to explore relationships between various environmental factors and soil microorganisms given their range of microclimatic conditions and vegetation types. This study investigated micro‐topographically (i.e., north‐/south‐facing slope aspects and flat plateau between them) controlled microbial diversity and community structures across a Lesotho mountain summit. Amplicon sequence analysis revealed that the north‐ and south‐facing slopes were dominated by more Proteobacteria and Bacteroidetes, while the plateau was dominated by more Acidobacteria. Fungi from the phylum Chytridiomycota more strongly dominated the plateau and the north‐facing slope than the south‐facing slope. Slope aspect, through its direct influence on air and soil micro‐climatology and plant diversity, significantly affects bacterial and fungal community structures at this location. These results provide original insight into soil microbial diversity in the Lesotho highlands and offer an opportunity to project the likely response of soil microorganisms to future climate warming in highly variable mountain environments such as the Lesotho highlands.

In the Southern Hemisphere (SH), north-facing slopes are oriented more directly toward the sun and receive greater intensity and duration of solar radiation than do opposing south-facing slopes (Codrington, 2005;Gilliam et al., 2014). High elevation north-facing slopes in the SH thus experience warmer climatic conditions with less shade and faster snow melt than south-facing slopes, especially during winter months (Gilliam et al., 2014;Jakšić et al., 2021).
Consequently, soils retain less moisture, and vegetation is generally more tolerant to warmer and drier conditions on north-facing slopes than on opposing south-facing slopes (Måren et al., 2015;Paudel & Vetaas, 2014). In contrast, south-facing slopes retain moisture for longer and are cooler, supporting more shade and moisturetolerant vegetation (Erdős et al., 2012;Pandita et al., 2019). These variations in species composition and richness of vegetation across mountain slope aspects also affect the quality and quantity of litter and organic matter available within the soil (Jakšić et al., 2021;Xue et al., 2018). Warmer north-facing slopes have less soil organic matter due to greater mineralization rates and sparse vegetation patterns, compared to the cooler south-facing slopes (Jakšić et al., 2021;Mohammad, 2008). This ultimately shapes and determines the number and type of soil microorganisms that inhabit mountain soils, causing either an increase or decrease in decomposition rates of organic matter and enzyme activity across a particular slope (Xue et al., 2018), underpinning Baas Bekings' statement "everything is everywhere, but, the environment selects" with regard to microbial diversity and biogeography (Baas-Becking, 1934).
Given that soil microorganisms are primarily responsible for processes such as nutrient cycling, decomposition of organic matter, and the formation of symbiotic relationships with higher organisms, these microorganisms are essential for ecosystem functioning and structure (Gilliam et al., 2014;Ma et al., 2004). While several studies determining the impact of mountain slope aspect on living organisms have focused on vegetation patterns (e.g., (Måren et al., 2015;Singh, 2018;Yang et al., 2020)), studies on the impact of (micro-) topography on soil microorganisms have only recently been forthcoming (Singh, 2018;Zeng et al., 2014). Researchers in several mountain ranges across China, Israel, New Zealand, and Europe have indicated that slope aspect plays a significant role in altering bacterial and fungal community structures (Adamczyk et al., 2019;Chu et al., 2016;Liu et al., 2017;Moroenyane et al., 2020;Wu et al., 2017;Xue et al., 2018). Some studies have shown bacterial community composition to be greater across warmer slopes, while fungal community composition is greater on cooler slopes (Chu et al., 2016;Wu et al., 2017). In contrast, other researchers have found a greater bacterial community composition across cooler slopes and greater fungal community composition across warmer slopes (Liu et al., 2017;Moroenyane et al., 2020). Despite such contradictory findings, differences noted in community composition have been linked to differences in vegetation patterns, soil properties and conditions, and environmental factors caused by slope aspect, all of which play a crucial role in shaping soil microbiomes (Chu et al., 2016;Liu et al., 2017;Moroenyane et al., 2020;Wu et al., 2017).
Mountain environments and their inhabited species are severely impacted by climate warming, and thus, it is the cause for concern (Sauer et al., 2011). Climate warming in these environments has already caused mean annual temperatures to increase 1.24 times faster in regions >500 meters above sea level (m.a.s.l.) compared to lower lying regions (i.e., <500 m.a.s.l.) over the period 1961-2010 (Wang et al., 2020). Soil microorganisms on mountains react to such climate warming through various responses, including changes in phenology, physiology, and community structure and distribution across habitats (Di Nuzzo et al., 2021;Sauer et al., 2011). Increasing temperatures also lead to an upward-elevational shift in suitable habitats, thereby causing compositional changes in species of soil microorganisms across mountains (Elsen & Tingley, 2015). However, due to topographic and land cover constraints toward mountain summits, the risk of extinction of soil microorganism species increases as suitable habitat availability decreases and competition for resources increases (Di Nuzzo et al., 2021).
To date, little is known about the influence of topography and associated microclimates on soil microbial diversity in southern Africa's highest mountain region, the Drakensberg (Figure 1). To this end, we explore bacterial and fungal diversity across a high (~3350 m.a.s.l.) mountain peak located near Kotisephola Pass in the Drakensberg, Lesotho, with the specific aim to establish any possible association between slope aspect-controlled soil temperatures/humidity and the overall microbiome. We aimed to test the following hypothesis: due to variations in vegetation zonation and microclimates associated with topographic location (i.e., slope aspect), bacterial and fungal community structures will vary across small spatial scales over mountain summits.

| Study area
The mountainous kingdom of Lesotho is the highest region in southern Africa, located between the latitudes 28°30′ to 30°40′ S and the longitudes 27°00′ to 29°30′ E (Grab et al., 2017). Lesotho's total land area of 30,355 km 2 comprises mostly high mountains and deep valleys, with all its land located over 1400 m.a.s.l. (Roberts et al., 2013). The climate varies from semi-arid to sub-humid, with generally mild-cool and wet summers, and cold and dry winters (Grab & Nash, 2010). Frost and several light-to-moderate snowfalls occur during the cooler seasons from May to September (Grab et al., 2017;Sene et al., 1998).
Our study was conducted along a slope transect, which included north-facing (N), summit plateau (P), and south-facing (S) slope positions ( Figure 2a). The chosen summit is located near Kotisephola Pass, inland of the infamous Sani Pass which marks a border between South Africa and Lesotho (Figure 1). The region falls between the longitudinal coordinates 29°30′46 to 29°30′52 south and the latitude 29°13′09 east. Sampling was undertaken over an elevational range of 3330 to 3350 m.a.s.l. Three sampling sites (A, B, and C) were selected along P, N, and S, respectively (Figure 2c), providing nine distinct sampling points (Table 1). The three sampling sites (A, B, and C) along the P, N, and S, were, respectively, selected based on variations in topographic position (i.e., slope aspect) and associated differences in vegetation characteristics and microclimatic F I G U R E 1 Geographical location of the study site in Lesotho (map adapted from QGIS, n.d.).

F I G U R E 2
Photographic and geographic layout of the study site in Lesotho. (a) Three sampling locations across the summit. (b) Layout of the rock scarp across the south-facing slope. (c) Distribution of the sampling sites across the plateau and the north-and south-facing slopes. The image also provides a representation of rock-scarp morphology along the north-and south-facing slopes (map images adapted from Google Earth, n.d.).
conditions. The underlying basaltic lithology is uniform across the slopes.

| Soil sample collection
The three primary sampling areas along the north-facing slope, plateau, and south-facing slope were distinct in terms of vegetation growth and patterns, microclimatic conditions, slope topography (rock scarp), and slope aspect. The south-facing slope has a relatively high (~5-6 m) and near vertical (80-90°) rock scarp, which results in long-lasting shade on slopes immediately below the scarp, and consequently permits snow accumulation through a snow-fencing effect and also preservation through protection from direct insolation.
Snow patches may last from late May to early October (Figure 2b), but is variable between years. Vegetation closest to the rock scarp (SA ~5.50 m distance) includes low Helichrysum shrubs, which then gradually incorporate a higher abundance of tall Merxmuellera drakensbergensis tussock grass with increasing distance downslope from the rock scarp (SB ~13.20 m and SC ~26.80 m distance). In contrast, the north-facing rock scarp is more shallow in gradient (50-60°) and does not provide for sharing on the slope below. Here, snow longevity is limited to 2 or 3 days post snowfalls.
The plateau is exposed to enhanced levels of solar radiation with no rock scarp present to provide shade. The plateau was sampled 19.82 m (PA) away from the southern end of the summit edge (SA) and 52.50 m (PC) distance from the northern end of the summit edge (NA) (Figure 2c). PA was located 57.50 m distance from PB, and PC was located 17.98 m distance from PB. Regolith cover was deepest (>0.5 m) on the south-facing slope, of intermediate depth   April, 2020. Data from the iButtons were retrieved using ColdChain

| Soil climate data collection
ThermoDynamics v 4.9 (Fairbridge Technologies, Sandton, SA) and subsequently analyzed for comparison with soil microbiome data.

| DNA extraction, PCR amplification, and highthroughput sequencing
Total soil DNA was extracted from ~0.4 g soil aliquots using the MoBio Powersoil DNA isolation kit (Qiagen), as per the manufacturer's instructions. DNA was quantified using the Nanodrop (Thermo Fisher Scientific) and was stored at −4°C until further use.
From the four replicates of soil samples collected at each of the nine sampling locations, a total of three random sample replicates per sampling location were selected to achieve a final batch of 27 sam-

| Sequence processing
The raw sequence data were processed using the MR DNA analysis pipeline (www.mrdna lab.com). Initial processing of the reads included joining of paired-end reads, removal of barcodes, removal of reads less than 150 base pairs in length, quality filtering of trimmed sequences, and denoising of sequences in which sequence errors were rectified. Subsequently, chimeric sequences were removed using UCHIME (Edgar et al., 2011). The trimmed and filtered reads were clustered according to their Operational Taxonomic Units (OTUs) on the basis of 97% sequence identity using USEARCH (Edgar, 2010). The OTUs were taxonomically classified using BLASTn against a curated database which was derived from the Ribosomal Database Project (www.rdp.cme.msu.edu) and NCBI (www.ncbi. nlm.gov).

| Statistical analysis
The processed sequences were analyzed using R v 3.6.1 (R Foundation for Statistical Computing; http://www.R-proje ct.org) and RStudio (RStudio Team, 2021). First, non-bacterial and non-fungal OTUs were removed from the dataset after which singleton sequences were removed, and the datasets were rarefied to standardize sample sizes (Valverde et al., 2016). Prior to rarefaction, two fungal sample replicates, SB2 (7599 sequences) and NC3 (19,403 sequences), were excluded from the analysis due to low numbers of sequences before singleton removal. The bacterial 16S rRNA reads were rarefied to 22,818 reads, whereas fungal ITS sequences were rarefied to 45,378 reads, representing the minimum number of sequences remaining in a single sample after the removal of low abundance counts.
Bacterial and fungal α-diversity was estimated using observed OTU richness and the Shannon-Weaver diversity index. Data distribution of α-diversity indices was determined using the Shapiro-Wilk test for normality. The relationship between these α-diversity indices and the geographical factor of slope aspect (categorical data) was then tested using the Kruskal-Wallis test. The Kruskal-Wallis test which is the non-parametric equivalent test of analysis of variance (ANOVA) and Wilcoxon rank-sum test with p-values adjusted using the false discovery rate (fdr) correction method were performed for bacteria (non-normally distributed) and ANOVA and Tukey's Honest Significant Difference test (Tukey's HSD) for fungi (normally distributed). The relationship between the α-diversity indices and abiotic (soil temperature and RH) factors (continuous data) was tested using a general linear model following Quasipoisson distribution and the Gaussian distribution for bacteria and fungi, respectively, using the function glm from STAT (R Core Team, 2021). The relationship of interactions between the abiotic (soil temperatures and RH) and geographical factor (slope aspect) on bacterial and fungal α-diversity indices was also determined using general linear mixed models by applying the function glm.
Bacterial and fungal β-diversity was analyzed using Bray-Curtis dissimilarity based on the relative abundance of OTUs. The impact of abiotic and geographical factors on community structures was determined using permutational ANOVA (PERMANOVA) with 1000 permutations. Pairwise β-diversity indices were calculated using the vegdist function and then the adonis function from vegan (Oksanen et al., 2020). Similarities between community structures were further visualized using non-metric multidimensional scaling (nMDS) with Bray-Curtis distance. The dispersion of the groups was further determined using the betadisp function.
Differential abundance testing was conducted using the DESeq function from DESeq2 (Love et al., 2014). The targets used included the plateau and the south-facing slope, while the north-and south-facing slopes were set as the base levels for comparisons, respectively.

| Soil microclimatic conditions
Temperature data across the slopes indicate that the south-facing slope was significantly colder than the north-facing slope and the plateau, both annually and seasonally (Table A1) Analysis of RH for soils across the slopes indicated that the south-facing slope had a significantly higher mean RH (99.74%) than the north-facing slope (88.69%) and plateau (85.07%), annually and seasonally ( during late summer (February) and early autumn (April) of 2020.
Such abnormally high RH readings are associated with the response of iButtons to stressors as iButtons function normally at a range of 50-100%. Stressors include the formation and deposition of water films over the device during periods of supersaturation through occurrences of heavy rain and snowfall.

| Taxonomic classification of bacterial and fungal communities
The amplicon datasets derived from soil samples were first rarefied to a total of 22,818 and 45,378 reads for bacteria and fungi, respectively. These reflected the lowest counts present in sample PB1 (center of plateau) for bacteria and SB1 (middle of south-facing slope) for fungi, respectively. Rarefaction curves generated for each of the samples for both the bacterial and fungal counterparts gradually reached a leveled state, which indicates that the process used to standardize sample sizes preserved the integrity of the samples ( Figure A1). This represents the structure of all fungal and bacterial communities found in soils across the three slopes.
These numbers represent the average percentages of bacterial phyla across all nine sites on the plateau and the north-and south-facing slopes. However, when comparing the different slope aspects across the summit, differences in phyla abundance are observed (Figure 3a). The microbial communities on the southand north-facing slopes incorporated proportionately more Proteobacteria (10.68% and 9.95%) than on the plateau (7.28%).
By contrast, the plateau included a greater relative abundance of Acidobacteria (7.75%) and Verrucomicrobia (6.26%), compared to the north-(6.93% and 4.75%) and south-facing slopes (6.06% and 4.35%), respectively. As with the bacteria, differences are noted between the sampled slope aspects (Figure 3b). The plateau fungal microbiome incorporated a higher relative abundance of Ascomycota (17.27%) and Chytridiomycota (7.46%), compared to the south-(17.02% and 6.22%) and north-facing slopes (14.80% and 7.42%), respectively. The south-facing slope lacked representatives of the phylum Monoblepharidomycota and a very low abundance of Blastocladiomycota (0.01%), compared to the north-facing slope (0.01% and 0.22%) and the plateau (0.02% and 0.23%) where these were only found in certain replicates. Mucoromycota and Neocallimastigomycota abundances were marginally higher on the north-facing slope (0.10% and 0.53%) compared to the south-facing slope (0.04% and 0.44%) and plateau (0.02% and 0.24%), although all these phyla were found at relatively low abundance on each aspect.

| Bacterial and fungal α-diversity
To evaluate the diversity of bacterial and fungal species within each of the montane sampling sites, observed OTU richness and Shannon-Weaver indices were determined (Table A2). Relatively greater bacterial diversity could be observed for the north-facing slope compared to the south-facing slope and plateau (Figure 4b).
For both the north-and south-facing slope, the highest diversity Apart from the middle of the south-facing slope (SB), the lowest fungal diversity was observed across all three sampling points along the plateau (Figure 4d). Fungal diversity was also greater on the north-facing slope compared to the south-facing slope and plateau.
In contrast to bacterial diversity on the north-facing slope, the middle portion (NB) mid-way from the rock scarp recorded the greatest fungal observed OTU richness, followed by the top (NA) and lower site (NC) sampling points which were closer toward and further away from the rock scarp, respectively. In contrast, fungal observed OTU richness decreased down the south-facing slope (from SA to SC) with increasing distance from the rock scarp (Figure 4c).
In contrast to the bacterial counterpart, the Shapiro-Wilk test used for fungal observed OTU richness (W = 0.93, p-value = .080) and Shannon diversity (W = 0.96, p-value = .325) indicated that these data were not significantly different from normality, and thus fol- and MARH in soil (df = 2, F-value = 2.518, p-value = .088).

F I G U R E 4
Variation in α-diversity indices for bacterial and fungal communities on the sampled mountain. Boxplots represent the observed OTU richness and Shannon-Weaver diversity indices for the different sampling points for the bacterial (a and b) and fungal counterparts (c and d), respectively. North A and South A were located at the top of the mountain followed by North B and South B in the middle and North C and South C at the bottom of the mountain slope. Plateau A represents the edge on the southern end Plateau C the edge on the northern end of the mountain, with Plateau B in the middle. Each sample point incorporates three replicates at each site, with the exception of NC and SB for fungi, where one replicate was eliminated due to low read counts.

| Bacterial and fungal β -diversity
The differences between bacterial and fungal soil microbial communities across the different slope aspects were determined using PERMANOVA based on Bray-Curtis dissimilarity. Bacterial community structure was significantly affected by slope aspect, MARH in soil, and distance of sampling locations from the rock scarps on the north-and south-facing slopes independently (Table 2). Slope aspect, however, was found to have a greater influence as it accounted for higher variance (R 2 ) compared to the distance of sampling locations from the rock scarps on the north-and south-facing slopes. Likewise, the effect of the interaction of MAST and MARH in soil with slope aspect and distance of sampling locations from rock scarps on the north-and south-facing slopes on bacterial communities was less than that of slope aspect itself. These interactions were also found to have no significant influence on bacterial community structures on the summit.
Fungal community structure, like its bacterial counterpart, was significantly affected by slope aspect, MARH in soil, and distance of sampling locations from rock scarps on the north-and southfacing slopes independently (Table 2). Likewise, slope aspect had a greater influence with a higher variance compared to the distance of sampling locations from the rock scarp on the north-and southfacing slopes independently, as determined using PERMANOVA.  Note: Table 2 shows the F-ratios for the test conducted to assess the effects of abiotic (soil temperatures and relative humidity) and locational attributes (slope aspect and distance from rock scarp) on bacterial and fungal community structures. df indicates the degrees of freedom, and R 2 indicates the proportion of variation explained by the factors. Significant p-values are indicated in bold. Permutations were taken at 1000.

TA B L E 2 Effects of abiotic and geographical factors on bacterial and fungal community structures.
Abbreviations: MAST, mean annual soil temperature; MSRH, mean annual relative humidity.

| Bacterial and fungal indicator taxa
As the slope aspect exhibited the strongest effect on bacterial and fungal communities, differential abundance testing using the DESeq2 approach was conducted to identify specific OTUs associated with Chytridiomycota (20 OTUs, mainly from the genus Rhizophydium) were more prevalent across the vegetated soils of the north-and south-facing slopes (log 2 FoldChange < 0) compared to the plateau (16 OTUs and 14 OTUs), respectively (log 2 FoldChange > 0) ( Figure 6c). Taxa from the phyla Basidiomycota (14 OTUs) and Glomeromycota (9 OTUs) were also more prevalent on the warmer north-facing slope (log 2 FoldChange < 0) compared to the southfacing slope (9 OTUs and 1 OTU), respectively (log 2 FoldChange > 0) ( Figure 6d). Taxa from the phylum Ascomycota had the greatest prevalence compared to all the indicator taxa and were distributed widely across the north-and south-facing slopes (142 OTUs, mainly from the genus Sphaerosporella) and the plateau (84 OTUs, mainly from the genus Mycoshaerella) (Figure 6c).

F I G U R E 5
Bacterial and fungal community structures across all sample sites for the three montane aspects. nMDS plots represent (a) bacterial and (b) fungal community structures across the montane aspects based on Bray-Curtis distance. The distance between points is indicative of the relative dissimilarities in community structures, with the ellipses indicating dispersion regions of each site. Bacterial stress solution was reached at about 0.16 and fungal at 0.21, both of which were relatively low.

| DISCUSS ION
The Lesotho highlands have been the focus of a broad range of geoand bio-environmental studies covering disciplines, such as geology, geomorphology, botany, climate, hydrology, and environmental change (e.g., Carbutt & Edwards, 2004;Fitchett et al., 2016;Grab & Knight, 2018;Mapeshoane & van Huyssteen, 2016;Norström et al., 2018). However, to date, there has been no attention given to microclimate and its control on biodiversity in these mountains, and studies on soil microbial diversity and community structure have been completely absent in the region. To this end, we have here presented a metataxonomic study, with a view to establishing the impact of varying slope aspects on bacterial (16 S rRNA) and fungal (ITS) soil communities across a summit in the Lesotho highlands. A total of 1,750,536 sequences were obtained from the summit, with fungal taxa accounting for 65% of these.
Slope aspect of mountainous terrain is known to significantly control environmental and microclimatic conditions such as temperature and moisture, among a host of other soil properties (Wu et al., 2017). At our Drakensberg site, the three slope aspects of the selected summit significantly affected soil temperatures and RH, as noted between May 2019 and April 2020. Variations in soil temperatures and RH across the slopes are attributed to microsite differences in precipitation, snow cover accumulation and longevity, and exposure to insolation (Boelhouwers & Meiklejohn, 2002;Grab, 1999;Mills et al., 2009). Temperature and RH recorded for soils across the three slope aspects in our study were also likely affected by the presence of snow cover, in particular below the south-facing rock scarp where snow tends to last longer (Grab et al., 2017).
Abnormally high RH recordings across the plateau and the north-and south-facing slopes can be attributed to the formation of water films over the iButtons during periods of supersaturation associated with high precipitation events or rapid snow melt (Cáceres et al., 2007). The lasting occurrence of snow below the rock scarp at higher elevations on the south-facing slope is likely the reason for the near-constant temperature ranges and abnormally high RH readings between the winter months of May-August 2019. Snow accumulation further insulates soils from freezing temperatures, thereby protecting above-and below-ground biota from extensive freezing during colder winter months (Contosta et al., 2019;Freppaz et al., 2007). The absence of shading and increased levels of solar radiation exposure on the plateau and the north-facing slope would have contributed to rapid snow melt and greater soil exposure, hence a greater range in temperatures and RH recorded along such topographic portions of our transect. Such inferences are supported by previously published arguments Grab et al., 2017).
The most common bacterial phyla found on the plateau and the north-and south-facing slopes included Acidobacteria, Actinobacteria, Bacteroidetes, Proteobacteria, and Verrucomicrobia, which accounted for more than 80% of all the bacterial phyla on the F I G U R E 6 Bacterial and fungal phyla identified as indicators on the slope aspects. Dot plots represent the most abundant significant indicators (p-adjusted < .050) of bacterial (a, b) and fungal (c, d) phyla classified at genus level. The target factors included the plateau (a, c) and south-facing slope (b, d) in comparison to the base levels which included the north-and south-facing slope and the north-facing slopes, respectively. Prevalence of particular taxa for the target factors is represented by log 2 FoldChange > 0, in contrast to prevalence of particular taxa for the base levels is represented by log 2 FoldChange < 0. The dots represent the number of indicator OTUs present of each genus.
summit. These bacterial phyla also dominated soils across cryic biomes which are typical to high mountain regions of the world, including the Changbai and Shennongjia Mountains in China, the Italian Alps, and the Tibetan Plateau (Shen et al., 2013;Siles & Margesin, 2016;Yuan et al., 2014;Zhang et al., 2015).  (Ho et al., 2017;Tada et al., 1995;Yao et al., 2017), whereas copiotrophic microorganisms such as Proteobacteria require nutrient-rich environments to survive (Koch, 2001;Yao et al., 2017). This compares favorably with our observations where the nutrient-poor plateau is associated with generally desiccated thin soils and exposed bedrock with little to no plant cover, while the north-and south-facing slopes had a diverse range of vegetation and soil coverage, accounting for more nutrientrich micro-environments.
The fungal microbiome across the summit mostly comprised of the phyla Ascomycota, Basidiomycota, Chytridiomycota, Unclassified fungal taxa, and Glomeromycota, which accounted for 98% of all phyla. The presence of these fungal phyla has similarly been reported from the Taibai Mountain in China, montane forests in South America, and retreating glaciers in the North American Arctic Transect (Geml et al., 2014;Ren et al., 2018;Timling et al., 2014).
Fungi are known to play several essential roles in soil, such as nutrient cycling, decomposition of organic matter, and forming symbiotic relationships with plant species (Krishnamoorthy et al., 2015;Schoch et al., 2009;Wang et al., 2020). Alterations in the diversity and composition of fungal communities due to changes in environmental and climatic conditions could thus have an impact on ecosystem functioning and vegetation establishments in soil (Adamczyk et al., 2019). As with bacteria, variations in the abundance of these fungal phyla across the three slope aspects were noted. The soil microbiome of the plateau and northfacing slope incorporated a greater abundance of Chytridiomycota, Basidiomycota, and Glomeromycota, compared to the south-facing slope. However, indicator OTUs from these phyla appeared to be greatly associated with the north-and south-facing slopes, compared to the plateau. Ascomycota prevailed across all three slope aspects in terms of relative abundance, as well as indicator OTUs.
As with bacteria, variations can be attributed to lifestyle strategies, with certain saprotrophic fungi, such as Chytridiomycota, likely possessing characteristics of copiotrophic microorganisms due to their need for nutrient-rich environments for decomposition (Ho et al., 2017;McConnaughey, 2014). In contrast, Glomeromycota and Basidiomycota may be classified as oligotrophic microorganisms (Ho et al., 2017). The presence of these phyla corresponds with their ability to survive in less vegetated and bare soil environments such as Drakensberg plateaus, as well as on the more vegetated north-and south-facing slopes. The presence of copiotrophic microorganisms, including Chytridiomycota, on the plateau, may be indicative of other potential drivers of such phyla in soils across this region. These include but are not limited to vegetation types, soil temperatures, and soil properties such as nutrient composition, pH, and carbon and nitrogen compositions which are known to influence soil fungal communities (Zhou et al., 2021).
Members of the phylum Ascomycota may be classified as either copiotrophic or oligotrophic microorganisms, regardless of them being saprotrophic (Yao et al., 2017), such that the isolated genera Fusarium and Penicillium are oligotrophic microorganisms, while Alternaria and Cladosporium are copiotrophic microorganisms (Ho et al., 2017). This ability of various taxa within Ascomycota to utilize generalized lifestyle strategies stems from the capability of this phylum to better withstand environmental stressors and use a large variety of resources, thus allowing them dominate soils in greater abundance (Egidi et al., 2019). increases (Kerns et al., 2018). Species dependent on cooler habitats are more prone to shift to thermally suitable habitats over short distances across mountains (Scherrer & Körner, 2011). Furthermore, recent decadal-scale changes in snowfall patterns, as is evident for the Lesotho highlands (Grab et al., 2017), may exert considerable influence on microbial species composition due to changes in the length of vegetation growing seasons (Scherrer & Körner, 2011). Ecological disturbances, which are expected in a warming climate, will inevitably affect microbial and plant species distribution, and thereby facilitate the transition to new community compositions of microbial and plant species (Kerns et al., 2018). With an ongoing warming scenario, it is assumed that: (1) microbial diversity and distribution on the south-facing slope may gradually mirror those that are currently present on the north-facing slope, and (2) the north-facing slope microbial diversity and abundance may gradually decrease and more strongly mirror that of the plateau.
Apart from slope aspect, the summit in our study included the presence of a rock scarp along the south-facing slope, thus contributing to notable microclimatic and vegetational zonation on the slope below. This topographic situation itself provides for the somewhat "unique" microclimate and environment below the rock scarps, hence further strengthening the slope-aspect control on bacterial and fungal diversity and community composition.

| CON CLUS IONS
Our study provides the first investigation of soil microbiome characteristics in the Drakensberg and Lesotho highlands of southern Africa. Differences in soil microbiomes over exceptionally complex but small spatial scales are attributed to topographic (slope aspect) and associated microclimatic controls. The soils across the summit in this study were found to contain highly diverse bacterial and fungal communities, despite the highly variable environmental conditions that were exhibited across the three slopes. Bacterial communities across the three slopes were predominated mainly by members of the phyla Acidobacteria, Proteobacteria, and Verrucomicrobia, while fungal communities were predominated by members of the phyla Ascomycota, Basidiomycota, and Chytridiomycota. Several bacterial and fungal phyla observed in this region are related to essential ecosystem processes and functions. The abundance of specific members of these phyla can thus be used as indicators of possible changes in environmental conditions based on the lifestyle strategies followed and essential environmental factors that act as drivers of changes in community composition. Slope aspect had a greater impact on bacterial and fungal diversity and community structure than did the distance of sampling location below a rock scarp. Despite this finding, the examined influence of the rock scarp provides new insight into previously unknown topographic controls affecting micro-abiotic dynamics and associated soil microbiome characteristics. Overall, our research approach and findings from current warmer slopes provide the potential to project future changes on current cooler slopes, given assumed climate warming scenarios. We hope such a methodological design sets an example of how the "natural laboratory" might be used to project future microbiome changes within complex topographies subjected to climate warming.

ACK N OWLED G M ENTS
The authors thank the South African National Research Foundation (NRF) for fully funding this research and for student funding (JP: NRF Masters Scholarship SFH180517331412).

FU N D I N G I N FO R M ATI O N
This research was supported by the South African National Research Foundation (NRF). Author JP has received the NRF Masters Scholarship SFH180517331412.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors have no relevant financial or non-financial interests to disclose.

O PE N R E S E A RCH BA D G E S
This article has earned Open Data and Open Materials badges. Data and materials are available at https://doi.org/10.5061/dryad.v6wwp zh0j.

DATA AVA I L A B I L I T Y S TAT E M E N T
Raw data (fasta sequence sets and OTU tables) are available for download from Dryad via the https://doi.org/10.5061/dryad.v6wwp zh0j. Any additional data pertaining to the study can be requested from the corresponding author.